This invention relates to cement compositions having zero free water content upon setting and a method for cementing well bores utilizing such compositions.
Cement compositions are used in the oil and gas industry to cement the annular space in well bores between the surrounding formation and pipe or casing. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. When the cement column so formed is allowed to stand, the cement particles, being of greater density, start to settle to the bottom of the slurry. As the particles settle, they tend to collect or "bridge." Just below any area of bridging, a crack is left which contains no cement particles. At the same time, water begins to accumulate at the top of the column. This accumulation of water at the top of the column of cement is known as "free water." Free water is undesirable because the formation of cracks or channels within the cement can lead to failure of the cement column under pressure.
In its specifications for class G and H cements, the American Petroleum Institute (API) Specification 10A, dated April, 1979, sets 1.4 percent by volume as the maximum allowable free water content. Even this amount of free water is detrimental, however. In addition to causing channels or cracks, free water pockets prevent bonding of the cement to the casing and formation and cause density variations in the set column. Free water can cause premature gelling of the cement column, lower the hydrostatic head on a gas zone, and allow gas migration into the column.
Prior art proposals to control excessive free water content call for the addition of certain natural and synthetic materials having a high attraction for water. Thus, the addition of bentonite clay has been proposed to bring the free water content of the composition to within API limits. Other materials proposed in the past include attapulgite clay, sodium metasilicate, diatomaceous earth, sodium silicate, polymers such as hydroxyethyl cellulose and carboxymethyl hydroxyethyl cellulose, and natural and synthetic pozzolans.
Certain of the prior proposals have reduced the free water content of cement compositions to within the API maximum allowable limits. However, even small percentages of free water can cause problems with the ultimate cementing job as has been discussed. Other prior art proposals utilized ingredients which were detrimental to other properties of the cement slurry at ingredient levels necessary to eliminate free water entirely.